The goal is to make sure that when someone double clicks the field, the autocomplete list from the browser doesn't come up.

The problem is, Google/Chrome, in their infinitely arrogant wisdom, decided to change the way this works, and instead of "off" being the right attribute value to disable autocomplete, "false" is (https://code.google.com/p/chromium/issues/detail?id=370363).

So... what's neded is for the Rich Widgets Auto Complete widget to be updated to do a browser detection, and use autocomplete="false" on Chrome and "off" on everything else.

As a workaround, would it be possible to add it in the extended properties? This assuming that browsers ignore the wrong value, so that if both "autocomplete="off"" and "autocomplete="false"" are specified, it picks the right one.

That was the first thing I tried. Doesn't work, because the Auto complete widget sets it to "off" anyways. :(

You would need to write your own browser detection script and use it to set the value to "false" in Chrome, and run it after the Auto Complete is added AND every time the Auto Complete gets refreshed/rerun.